The present invention relates to a method for the modulation of the signal power in optical transmission systems comprising at least one optical amplification device and to a method for the detection of bit errors in the processing of digitalized transmitted data, whereby the data are transmitted by an optical transmission system.
Optical communication systems are a fast-growing constituent of communication networks. The term “optical communication system” as used in the following relates to any system or device which makes use of optical signals to transport information across an optical waveguiding medium. Optical communication systems comprise inter alia telecommunication systems, local area networks (LAN), cable television systems etc.
For the transmission capacity of optical fibres in optical communication systems is expected to advance in the future, the evolution of optical amplification is one of the core technologies involved in this process. A key to this evolution is the availability of extremely-broad-band optical amplifiers, offering amplification over nearly all the transmission window allowed by silica. These requirements can be met inter alia by Raman amplification.
Optical fibre Raman amplifiers (FRA) are well known and are known to be designed to operate at a desired wavelength between 1.25 μm and 1.7 μm. FRA utilize silica-based fibres and display a high transparency when unpumped. The working principle of FRA is based on stimulated Raman scattering as for example explained in the Ph.D. thesis of P. Riedel with the title “Untersuchungen zum künftigen Einsatz solitonengestützter faseroptischer Nachrichtenübertragung bei 1,3 μm Wellenlänge”, Hamburg 1998, the disclosure of which is incorporated herein by reference. FRA can serve for example as a replacement for conventional repeaters or semiconductor-amplifiers, or for rare-earth-doped fibre amplifiers or in combination with them.
The transmission capacity of optical fibres is expected to advance in the future with the objective of reaching 10 Tbit/s capacities, and over. However, even with an extremely good spectral efficiency of 0.8 as in some amplification arrangements disclosed in the prior art, the useful bandwidth that is required to stuff a transmission capacity of 10 Tbit/s is in excess of 12.5 THz.
A key to this evolution toward capacities over 10 Tbit/s is the availability of extremely-broad-band optical amplifiers, offering amplification over nearly all the transmission window allowed by silica. These requirements can be met inter alia by Raman amplification.
Raman amplification allows to extend the transmission to wavelengths which are not addressed by fibres doped with rare-earth elements, for example Erbium or Thulium. This means, that Raman amplification opens the possibility to have optical transmission systems either in the 1.3-μm transmission window as well as in the 1.5-μm transmission window, or even in the 1.4-μm and 1.6-μm wavelength regions.
Usually, the approximate values for the bandwidths of the different “bands” as explained in the foregoing as well as in the following can be summarized as follows:
TABLE 1XS-bandS-BandC-bandL-Band1270 to 13501450 to 15251525 to 15651565 to 1610nmnmnmnm230 to 228204 to 199196 to 191191 to 186THzTHzTHzTHz
Bandwidths larger than approximately 1605 nm are conventionally termed as “XL-band”.
The bandwidth values in table 1 are given in nm and in Hz since channels of optical systems are spaced in frequency.
Raman tilt, however, is extremely intense in very-wide-band transmissions, for example in the combined C+L band, and even more in the combined C+L+XL bands. As a consequence, any important fluctuation in signal power (many channels in a sub-band which are simultaneously at 0 or 1) would significantly alter the system performance.
The power in each channel, or group of channels, covering approximately 1 THz should thus be kept constant over time, on a time scale in the order of 10 bit times or even less (about 10 bit times for a 10 Gb/s per channel transmission over Alcatel TeraLight® fibre, about 5 bit times over other commercial non-zero dispersion shifted fibres).
Similar problems have been observed with Semiconductor amplifiers (SOA), which display a high cross gain modulation (CGM). Therefore a SOA does perceive any variation in the signal power, even on bit-time scale, i.e. problems arise from crosstalk. Kim et al. (OAA 2000, Quebec, paper OtuB3 and PDP2, 2000) proposed a wavelength modulation technique to reduce the CGM effect. This modulation technique is based on the constant channel power where the information is coded in a form of wavelength modulation which is implemented by using a dual input-single output Mach-Zehnder LiNbO3 modulator.
CGM, which results from spectral hole-burning is also expected to occur, though with a much longer time-scale, in rare-earth-doped amplifiers, or other types of amplifiers, thus degrading the system performance in the instance of very long sequences of “zeroes (0)” or of “ones (1)”.